Thermionic generator and amplifier of high frequency oscillations



Dec. 26, 1933. wRlGHT ET AL 1,940,986

THERMIONIC GENERATOR AND AMPLIFIER OF HIGH FREQUENCY OSCILLATIONS Filed June 5, 192

AND SIDNH BERTRNA SMITH Patented Dec. 26,1933 v 1 i ITED; PATENT OFFICE 7' i i #940,986 I i v 1 THERMIONIC GENERATOR AND AMPLIFIER I OF HIGH FREQUENCY OSQILLATIONS George Maurieewri 'ht, (lhesterfieldtand sianey Bertram Smith,helmsford,Englandmssignors 'to'RadioCorporation ofAme'ri'ca, a corporation of- Delaware Application. June 5,v 1-926, Serial No. 113,859, and in Great Britain June 29, 1925 I 9- Clair us. (Cl. 179-471 This invention relates to thermionic generations in a circuit passing: from one-tuned circuit, tors andamplifiersof high frequency oscillations, through one tube, the second tuned circuit, the and has for its'object thenullifying of the reacsecond tube andiback to thefirst tuned circuit,

, tion which normally takes: place between: the and that the balancing, condensers do not pre- 5 id; and plate circuits of. thermionic tubes emvent oscillations takingplaceinthis circuit. The 0-.

ployed generators and amplifiers of highfrearrangement is, in fact, in thisirespect not unquency oscillations, due to the natural Self-Car ik the u ual two" ube-o used for e pacity of the tube, generation of very short Waves.

In general; devices employed hitherto for this T a y Oseillatierls y also takeiipltwe iII l9 Purpose depend for their action upon the Wheat- Circuit passing from e d Point Of t fi 65" '30 According to-lthe present invention,1we.employ equal;

stone bridge principle, and in such devices the inductance through the W a ve of this i Wheatstone bridge or its equivalent is soarductance in parallel (a d pp s i threugh ranged that the grid and output circuits respecthe ty the tw t si parallel and t tively arearranged each atthe' opposite ends of halves of thesecond inductance'to'its midpoint, a diagonal of the bridge; the object being to pret n v a the i h tension r pp y a i 70.

vent a change of voltagein theone case fromLrenegative. z y I sulting'in a change of' voltage in the other. The In app y 1 invention to this amplifier, bridge is u'suallyprovided with additional'cap'acih a pi resistances are included e c in ties. to balance the natural self capacity of the s i sw tha d pr f a y n t t the ids of tube. Y the two tubes, and damping resistances are also Theoretically, if. such a bridge be balanced, no nn ted in series vwith h balancing reaction will take place, but-in practice it has dens rs, th r s stan s in s p p rt d been. found that none of thesearrangements beas not to ups h ba an e of th b d e-J haves as stablyxas the simple bridge theory Would We haveifoundthat resistances S0 inserted are 25 indicate. It is believed that'the instability is due ct ve in damping outboth thezsecondaryand 30, to the existence of secondary or tertiary oscillatertiary modes. of oscillation. v ti r hi h are ngt ala t byth dg In the case in which identical tubes are emarrangement. Such oscillations in general take p y that 1301 y tubes W natural place around the. bridge rather than across it; p y' is q all of the resistances m y b t v n I an arrangement; of the; Wheatstone bridge type, 1 It has beenfound, that such a circuitis stable. in a manner similar to that previouslyreferred The invention is described withreference to the to; andnwe' provide damping resistances in the accompanyingdrawing, in which Figures 11 and 7 bridge; such: that whilenot upsetting the bal- 2:,show diagrammatically twoforms of known 35 ance" thereof, they 'damp' out secondary; or teranti-reaction circuit arrangements; Figures 3, 4 v

' tiary' oscillations. a V and 5 are diagrammatic representations of the Theinvention islapplicable to various arrangearrangement s w in drawn t in 'ments, employing. thev Wheatstoneabridge prindifierent form to show the different modes of, osciple. It may, ior'example, be applied tothe socillation existing therein; Figure" 6' shows dia- '40 called push-pull type 1 of: amplifier, provided a a ye l y Of p y present 95,

= with meansfor; preventing reaction. One known inv ntion t0 e anti-reaction arrangement form: of such amplifier comprises attuned circuit sh wn in gu e 7 shows diagrammatically connected at each endto: the grid ofa; thermionic one Way pplyin h p -i ve t0 the tube, the anodes of "the two tubesa employed'beanti-reaction arrangement shownin Figure 1-.-

45 mg. connected; through a second tuned circuit. E efer g to Figurexl, this shows One known The mid. point of the inductance forming part type of bridge circuit; inthis case the'anode cirofwthe first tuned circuit forms the, grid-negative cuit inductance is tapped at the point X, the connection, and the mid point of the inductance saidpoint being connectedthrough the high ten- .forming part: ofthe second tuned circuit form 'sion. battery, and the tube filament to the-low 5O thehigh tension:positive'connection; potential, end of the grid circuit. B is they anti- Anti-reaction, balancing condensers are; conreaction balancing condenser. The portions XY nestedeach. between the plate of one thermionic and XZ form the ratio arms of the bridge and a tubea d=0 the he value can e found-for the condenser Bin rela- WeliaiVe ioundvthet despitethe balancing eifect tion to the grid.anodetube-capacity' suchthat the of; the condensers, there exist secondary ,oscillae. bridge is balanced and the desiredicondition obsults from the application of the principle of the push-pull connection of a pair o f tubes, as shown in Figure 2. In this case there are two anti-reaction condensers B1 and 132, which .are connected from the grid of each tube to the anode of its companion tube.

It has been found in practice that none of these arrangements-behaves as stably as the simple Wheatstone bridge theory would indicate. We

have found that the cause of instability is due.

to the existence of second and third modes of oscillation, which are not balanced out by the bridge connection.

The simplest circuit to consider is the sym-. metrical push-pull arrangement of Figure 2. If the, tubes be represented simply as condensers, and the anti-reaction condensers B1 and B2 be omitted, the circuit becomes that shown in Figure 3. r

In this figure L1 01' and L2 C2 are the grid and anode tuned circuits, V1 and V2 are the two tubes, while L3 represents the inductances of the connecting leads. It will be apparent from the figure that a second degree of freedom exists, the mode of oscillation being around the closed circuit formed by "C1 L3 V1 L3 C2 L3 V2 L3;

This circuit may be again redrawn, as shown in Figure 4.

It will be seen that this circuit is the same as that usually employed fora two tube oscillator for the generation of very short waves. The antireaction condensers are connected between OR QPJand it is obvious from inspection of the figure that the said anti-reaction condensers cannot balance out the reaction through the tubes for this mode of oscillation. If (71:02 and V1=V2 and the inductances represented by L3 are all equal, then from the symmetry of the figure, the points 0 and R and the points P and Q will be at the same potential when oscillations occur. Consequently connecting these pairs of points by condensers or any other type of impedance will have no eifect whatever upon the oscillations. 1

This second mode of oscillation is one cause of the instability of anti-reaction circuits of the foregoing type, and such circuits have a strong tendency to break into oscillation generally at short wave length.

The push-pull'circuit of Figures 2, 3 and'4 can be also redrawn as shown in Figure 5.

In Figure 5 the anti-reaction condensers B1 and B2 of Figure 2 have been omitted. It can be seen that a third mode of oscillation'exists round the circuit formed by the two tubes acting in parallel and the loop a b c are f. In this case, it will be seen that the anti-reaction condensers B1 and B2 (Figure 2) will not nullify this tendency to oscillate; in some cases it will even tend to exaggerate this mode of oscillation. This oscillation is usually ofshort wave length.

-We have found that the circuit can be made stable by damping the'oscillatory paths taken in these two modes of oscillation. One method of dampingthese oscillations is by inserting a series resistance between the high potential end of the grid inductance and the grid of the tube. The complete push-pull circuit then becomes that shown in Figure 6.

Referring to this figure, T1 and T2 are resistances of such value that these modes of oscillation are suppressed; such a circuit is" stable. Where the tubes employed are identical, the resistances T1 and 12 may be identical; if the circuit is unsymmetrical or the tubes differ in capacity, the resistances 1'1 and 7'2 should be adjusted to suit the special conditions.

In the case of a single tube circuit, such as that shown in Figure ,l, the damping resistances may be inserted as shown in Figure '7, 1'1 and 12 being the said damping resistances. In this casethe resistances must be so proportioned that the ratio of the tube capacity V0 to that of the balancing condenser E, is given by the relation Other anti-reaction circuits may be similarly damped. r 1

In certain cases, suchas audio'frequency amplifiers where balancing condensers are not essential for nullifying the first mode of oscillation, the second and third modes of oscillation may be eliminated by insertingrsuitable resistances between the tube. grid and the input impedance. J

In certain cases it may be an advantage to connect the damping resistancein the high potential end of the tube anode circuit; for example, if. the loss in amplification, due to. the voltage drop across a resistance connected'to the grid, is too large for satisfactory operation, a better efficiency may beobtained by using equivalent resistances in the anode circuit;

.Having described our invention, what we claim is:

l. A thermionic amplifiericomprising input and output circuits, a pair of vacuum tubes. op-

positely connected in said circuits, andsoarranged that one vacuum tube repeats certain parts of said' incoming wave, the other vacuum tube repeating the remaining part of. said incoming wave, a capacity between thev input and output circuit for each-0f said vacuum tubes and adapted to neutralize the grid-plate capacity thereof to preventgeneration of oscillations at signal frequency, and means fordamping oscillations at other frequencies in said circuits comprising a resistance in the input lead-of each :of said vacuum tubes. J":

2. In thermionic repeating circuits including a plurality of thermionic tubes in push pull arrangement having input and output circuits. associated therewith which inherently include primary, secondary, and tertiary oscillation .cir-

cults, each free to oscillate at its respective predetermined frequency, the combination of means for preventing oscillations in the primary oscillation circuit, means for damping oscillations in the secondary oscillation circuit, and means independent of the second means for .dampingoscillationsin the tertiary oscillation circuit;

3. In thermionic repeating circuits including a plurality of thermionic tubes in push pull arrangement having input and output circuitsassociated therewith adapted to oscillate at a primary frequency, the combinationl of capacity means for preventing oscillation in said input and output circuit at said primary frequency, said circuits including said capacity means inherently including secondary, and tertiary oscillation circuits each 'freeto oscillate at its respective predetermined frequency, and a resist.-

anoe contained in each of said secondary and tertiary oscillation circuits for damping oscillations therein.

' sociated therewith which inherently provide'pri-,

mary, secondary, and tertiary oscillation circuits each free to oscillate at a primary, secondary and tertiary frequency respectively, the combination of capacitive means in shuntwith a portion of said primary oscillation circuit for preventing oscillations at primary frequency therein, a resistance in series with said secondary oscillation circuit for damping oscillations therein, at said secondary frequency, and a resistance 'in series with said tertiary oscillationcircuit for damping oscillations therein at said tertiary frequency. 7

5. In thermionic repeating circuits including a plurality of thermionic tubes'in push pull arrangement having input and output elements and input'and output circuits associated therewith adapted to oscillate at a primary frequency, the combination of capacity means associated with each tube for preventing oscillation in said input and said output circuit at said primary frequency, ,said circuits including said capacity means inherently including secondary and tertiary oscillation circuits each free to oscil- I late at its respective predetermined frequency, and resistances in said secondary and tertiary oscillation circuits for damping oscillations there-' ,1 and a coupling between the input and output circuits for each of said tubes and effective to counteract the effect of retroactive currents through the control electrode-anode impedance of its respective tube, and an independent resistance in series with each of said impedances to prevent the generation of oscillations in the stage at frequen- Y cies above the frequencies of the currents 1m pressed on said input circuit.

'7. In apparatus of the character described, an electron discharge device having an anode a cathcuit connected between said cathode and control electrode, a resistance connected between said control electrode and parallel tuned input circuit, a parallel tuned circuit connected between the potential of said anode, whereby said device acts as an amplifier substantially free from spurious oscillation generation over a band of fre-; quencies.

8. In apparatus of the character described, a pair of electron discharge devices each having an anode a cathode and a control electrode, means for connecting the cathodes together, a reactance connected between said control grids, a reactance con-'- nected between said anodes, means for reactively connecting the anode of one of said devices to the control grid of another, means for reactively connecting the anode of said other device to the control grid of one device, and, a resistance 'in series with each of said control grids whereby said dea desired operating frequency and at other frequencies.

9. In apparatus of the character described, a pair of electron discharge devices, each having an anode a cathode and a control grid, a parallel tuned circuit connected between said control grids, a parallel tuned circuit connected between said anodes, a resistance connected between each of said grids and said parallel tuned circuit connected to said grids, a condenser connected between the anode of one of said devices and the control grid of another of said devices, and another condenser connected between the control'gri'd of said other device and the anode of said one device whereby ode and a control electrode, a parallel'tuned cirsaid resistances andcondensers act to prevent spurious oscillation generation. 

